TY - JOUR
T1 - Origin of Earth's Water
T2 - Chondritic Inheritance Plus Nebular Ingassing and Storage of Hydrogen in the Core
AU - Wu, Jun
AU - Desch, Steven
AU - Schaefer, Laura
AU - Elkins-Tanton, Linda
AU - Pahlevan, Kaveh
AU - Buseck, P R
N1 - Funding Information:
This work was mainly supported by the research grant “Water from the Heavens: The Origins of Earth's Hydrogen” sponsored by the W. M. Keck Foundation (P.I. Peter Buseck). The results benefited from collaborations and/or information exchange within NASA's Nexus for Exoplanet System Science (NExSS) research coordination network sponsored by NASA's Science Mission Directorate (grant NNX15AD53G and P.I. Steven Desch). We thank Adam Sarafian and an anonymous reviewer for their constructive comments and suggestions on an earlier manuscript. All the data sets required to create the figures and tables in this work can be generated using the FORTRAN codes that are included in the supporting information.
Publisher Copyright:
©2018. American Geophysical Union. All Rights Reserved.
PY - 2018/10
Y1 - 2018/10
N2 - Recent developments in planet formation theory and measurements of low D/H in deep mantle material support a solar nebula source for some of Earth's hydrogen. Here we present a new model for the origin of Earth's water that considers both chondritic water and nebular ingassing of hydrogen. The largest embryo that formed Earth likely had a magma ocean while the solar nebula persisted and could have ingassed nebular gases. The model considers iron hydrogenation reactions during Earth's core formation as a mechanism for both sequestering hydrogen in the core and simultaneously fractionating hydrogen isotopes. By parameterizing the isotopic fractionation factor and initial bulk D/H ratio of Earth's chondritic material, we explore the combined effects of elemental dissolution and isotopic fractionation of hydrogen in iron. By fitting to the two key constraints (three oceans' worth of water in Earth's mantle and on its surface; and D/H in the bulk silicate Earth close to 150 × 10−6), the model searches for best solutions among ~10,000 different combinations of chondritic and nebular contributions. We find that ingassing of a small amount, typically >0–0.5 oceans of nebular hydrogen, is generally demanded, supplementing seven to eight oceans from chondritic contributions. About 60% of the total hydrogen enters the core, and attendant isotopic fractionation plausibly lowers the core's D/H to ~130 × 10−6. Crystallized magma ocean material may have D/H ≈ 110 × 10−6. These modeling results readily explain the low D/H in core-mantle boundary material and account for Earth's inventory of solar neon and helium.
AB - Recent developments in planet formation theory and measurements of low D/H in deep mantle material support a solar nebula source for some of Earth's hydrogen. Here we present a new model for the origin of Earth's water that considers both chondritic water and nebular ingassing of hydrogen. The largest embryo that formed Earth likely had a magma ocean while the solar nebula persisted and could have ingassed nebular gases. The model considers iron hydrogenation reactions during Earth's core formation as a mechanism for both sequestering hydrogen in the core and simultaneously fractionating hydrogen isotopes. By parameterizing the isotopic fractionation factor and initial bulk D/H ratio of Earth's chondritic material, we explore the combined effects of elemental dissolution and isotopic fractionation of hydrogen in iron. By fitting to the two key constraints (three oceans' worth of water in Earth's mantle and on its surface; and D/H in the bulk silicate Earth close to 150 × 10−6), the model searches for best solutions among ~10,000 different combinations of chondritic and nebular contributions. We find that ingassing of a small amount, typically >0–0.5 oceans of nebular hydrogen, is generally demanded, supplementing seven to eight oceans from chondritic contributions. About 60% of the total hydrogen enters the core, and attendant isotopic fractionation plausibly lowers the core's D/H to ~130 × 10−6. Crystallized magma ocean material may have D/H ≈ 110 × 10−6. These modeling results readily explain the low D/H in core-mantle boundary material and account for Earth's inventory of solar neon and helium.
KW - D/H ratio
KW - Earth's water
KW - hydrogen isotope fractionation
KW - magma ocean
KW - nebular ingassing
KW - planetary embryo
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U2 - 10.1029/2018JE005698
DO - 10.1029/2018JE005698
M3 - Article
AN - SCOPUS:85055283578
SN - 2169-9097
VL - 123
SP - 2691
EP - 2712
JO - Journal of Geophysical Research: Planets
JF - Journal of Geophysical Research: Planets
IS - 10
ER -